The discovery of new materials with novel chemical and physical properties often leads to the development of new and useful technologies. Over forty years ago, for example, the preparation of single crystal semiconductors transformed the electronics industry. Currently, there is a tremendous amount of activity being carried out in the areas of superconductivity, magnetic materials, phosphors, nonlinear optics and high strength materials. Unfortunately, even though the chemistry of extended solids has been extensively explored, few general principles have emerged that allow one to predict with certainty composition, structure and reaction pathways for the synthesis of such solid state compounds. Moreover, it is difficult to predict a priori the physical properties a particular three dimensional structure will possess. Consider, for example, the synthesis of the YBa.sub.2 Cu.sub.3 O.sub.7-8 superconductor in 1987. Soon after the discovery of the La.sub.2-x Sr.sub.x CuO.sub.4 superconductor, which adopts the K.sub.2 NiF.sub.4 structure (Bednorz, J. G. and K. A. Muller, Z. Phy. B 64:189 (1986), it was observed that the application of pressure increased the transition temperature (Chu, et al., Phys. Rev. Lea. 58:405 (1987)). As such, Chu, et al. attempted to synthesize a Y--Ba--Cu--O compound of the same stoichiometry in the hope that substitution of the smaller element, i.e., yttrium, for lanthanum would have the same effect. Although they found superconductivity above 93K, no phase with K.sub.2 NiF.sub.4 structure was observed (Wu, et al., Phys. Rev. Lett. 58:908 (1987)). Even for the relatively simple intermetallic compounds, such as the binary compounds of nickel and zirconium (Ni.sub.5 Zr, Ni.sub.7 Zr.sub.2, Ni.sub.3 Zr, Ni.sub.2 Zr.sub.8, Ni.sub.10 Zr.sub.7, Ni.sub.11 Zr.sub.9, NiZr and NiZr.sub.2), it is not yet understood why only certain stoichiometries occur.
Clearly, the preparation of new materials with novel chemical and physical properties is at best happenstance with our current level of understanding. Consequently, the discovery of new materials depends largely on the ability to synthesize and analyze new compounds. Given approximately 100 elements in the periodic table which can be used to make compositions consisting of three, four, five, six or more elements, the universe of possible new compounds remains largely unexplored. As such, there exists a need in the art for a more efficient, economical and systematic approach for the synthesis of novel materials and for the screening of such materials for useful properties.
One of the processes whereby nature produces molecules having novel functions involves the generation of large collections (libraries) of molecules and the systematic screening of those libraries for molecules having a desired property. An example of such a process is the humoral immune system which in a matter of weeks sorts through some 10.sup.12 antibody molecules to find one which specifically binds a foreign pathogen (Nisonoff, et al., The Antibody Molecule (Academic Press, New York, 1975)). This notion of generating and screening large libraries of molecules has recently been applied to the drug discovery process. The discovery of new drugs can be likened to the process of finding a key which fits a lock of unknown structure. One solution to the problem is to simply produce and test a large number of different keys in the hope that one will fit the lock.
Using this logic, methods have been developed for the synthesis and screening of large libraries (up to 10.sup.14 molecules) of peptides, oligonucleotides and other small molecules. Geysen, et al., for example, have developed a method wherein peptide syntheses are carried out in parallel on several rods or pins (see, J. Immun. Meth. 102:259-274 (1987), incorporated herein by reference for all purposes). Generally, the Geysen, et al. method involves functionalizing the termini of polymeric rods and sequentially immersing the termini in solutions of individual amino acids. In addition to the Geysen, et al. method, techniques have recently been introduced for synthesizing large arrays of different peptides and other polymers on solid surfaces. Pirrung, et al., have developed a technique for generating arrays of peptides and other molecules using, for example, light-directed, spatially-addressable synthesis techniques (see, U.S. Pat. No. 5,143,854 and PCT Publication No. WO 90/15070, incorporated herein by reference for all purposes). In addition, Fodor, et al. have developed, among other things, a method of gathering fluorescence intensity data, various photosensitive protecting groups, masking techniques, and automated techniques for performing light-directed, spatially-addressable synthesis techniques (see, Fodor, et al., PCT Publication No. WO 92/10092, the teachings of which are incorporated herein by reference for all purposes).
Using these various methods, arrays containing thousands or millions of different elements can be formed (see, U.S. patent application Ser. No. 805,727, filed Dec. 6, 1991, the teachings of which are incorporated herein by reference for all purposes). As a result of their relationship to semiconductor fabrication techniques, these methods have come to be referred to as "Very Large Scale Immobilized Polymer Synthesis," or "VLSIPS.TM." technology. Such techniques have met with substantial success in, for example, screening various ligands such as peptides and oligonucleotides to determine their relative binding affinity to a receptor such as an antibody.
The solid phase synthesis techniques currently being used to prepare such libraries involve the stepwise, i.e., sequential, coupling of building blocks to form the compounds of interest. In the Pirrung, et al. method, for example, polypeptide arrays are synthesized on a substrate by attaching photoremovable groups to the surface of the substrate, exposing selected regions of the substrate to light to activate those regions, attaching an amino acid monomer with a photoremovable group to the activated region, and repeating the steps of activation and attachment until polypeptides of the desired length and sequences are synthesized. These solid phase synthesis techniques, which involve the sequential coupling of building blocks (e.g., amino acids) to form the compounds of interest, cannot readily be used to prepare many inorganic and organic compounds.
From the above, it is seen that a method and apparatus for synthesizing and screening libraries of materials, such as inorganic materials, at known locations on a substrate is desired.